Place the onion into the Instant Pot and saute. You’ll want to use a silicone spoon to stir while cooking. (I like this spoon because I like the size and feel of the handle, plus it washes very well.)
Add the ground beef into the pot and brown that.
Add the 24 oz jar of sauce, as well as 24 oz of water.
Add the ziti.
Set to steam for 5 minutes.
Do a quick pressure release.
Add the 4 oz shredded mozzarella.
Stir until cheese is melting, or consistency you like.